Studies of the L2C lymphatic leukemia of inbred strain 2 guinea pigs: Efforts are now being focused on the chemical nature of a soluble KCl extract containing a tumor specific transplantation antigen (TSTA) obtained from these cells. The active moiety has a MW of 20,000 and is basic. It does not appear to be related to the cell surface IgM. The material is heat resistant but is sensitive to trypsin, neuraminidase and periodate. These properties suggest that the TSTA is a low MW glycoprotein. We are investigating human suppressor T cells generated by concanavalin A activation. Such suppressor T cell activity is diminished in most patients with systemic lupus erythematosus (SLE). In addition, we have observed that in most patients with active SLE the autologous mixed lymphocyte reaction (auto-MLR) between T cells and non-T cells is also absent. The T cell responsible for auto-MLR and that which mediates suppressor function may be one and the same cell.